The brain includes gray matter connected by channels of nerve fiber, which is often white matter and sometimes referred to as the fasciculi. A purpose of tractography is to identify the paths, or tracks, followed by the nerve fiber and to use those tracks to form a map of a standard brain. By inspecting the resulting map, one might elucidate certain patterns and structures that may shed light on how the brain carries out its functions.
A physiological characteristic of the nerve fiber tracks is that water will tend to diffuse along the direction of those tracks and will tend to encounter resistance in attempting to diffuse transverse to the tracks. Thus, by observing the directions in which water diffuses at different locations in the brain, one can identify the directions of major fiber bundles within the brain tissue. If one could identify a preferred direction of diffusion, one would be able to determine the directions of the fiber bundles in that tissue.
A known way to collect data indicative of the diffusion of water in the brain is to obtain MRI images of the brain. One approach to doing so, known as “diffusion tensor MRI,” involves estimating a diffusion tensor at each voxel in the brain. Other approaches include representing the diffusion characteristic at each voxel with a orientation density function defined over a sphere or over another volume. Yet another approach is to represent the diffusion characteristic at each voxel. FIG. 1 provides a comparison between tractographic data from a structure as obtained using DSI and tractographic data from the same structure as obtained using the q-ball method.
In much of the brain, and in particular, throughout the cerebral cortex, these tracks tend to cross repeatedly. Thus, if one could accurately display every track, the resulting display would look much like a ball of steel wool, with millions of individual tracks that cross and re-cross each other to form an apparently seamless and featureless whole. Such a display would be difficult to interpret.